This invention relates in general to chemical etching of metals and, more specifically, to improvements in the etching of portions the interior walls of narrow metal tubes.
Chemical etching processes in which a pattern of etchant-resistant maskant is placed over a metal surface, the surface is immersed in an etching solution which dissolves the exposed metal to the desired depth, the surface is removed and rinsed and the maskant is removed have long been known. This has been found to be very effective in producing precise patterns and of removing excess metal to reduce the weight of structures, especially those used in aerospace products.
Weight saving is very important in aircraft to reduce the energy consumed in flight. Attempts have been made to chemically etch interior surfaces in aircraft engine shafts which require a high tolerance cylindrical exterior surface and greater strength near the ends, but less strength near the centers. Such tubular shafts have been dipped into a maskant (one or both ends) to leave a central area exposed. The tube is immersed in an etchant which etches the tube interior, significantly reducing weight. Unfortunately, this process has had a number of problems with very irregular edges on the etched area, poor fillets along the edges of the etched area and etch leaks through the maskant near the etched area.
Attempts have been made to control this problem by inserting a scribing tool into the tube and scribing a circular line through the maskant at the desired etch area edge. The maskant overlapping into the area to be etched is removed and etching is accomplished. While successful in overcoming the edge irregularity and poor fillet problems, this method is very difficult to accomplish with very narrow tubes and requires great skill in scribing the line and removing the excess maskant without damaging the remaining maskant.
With many metals, particularly with iron-nickel-chromium alloys, etching is especially difficult in the confines of a narrow tube, with uneven etching and channelling being likely to occur, producing a less than optimum etched surface. In aircraft engines or the like it is particularly important that clean etch lines with smooth fillets be produced to retain maximum tube strength together with maximum removal of unnecessary weight.
Thus, there is a continuing need for improvements in methods for etching away unnecessary tube wall thickness in specific areas within tubes intended for aerospace applications.